A tandem type image forming apparatus can include photoconductors, which are provided individually for respective colors (such as black, cyan, magenta and yellow), are arranged along the rotational direction of a paper conveyor belt, so that images of respective colors held on the photoconductors can be sequentially transferred to paper on the belt.
A resultant color image formed by the tandem-type image forming apparatus may include a color shift, due to displacement of images of respective colors from one another. In view of this, some of image forming apparatuses have a function for aligning the forming positions of images of respective colors.
During the alignment function being performed, the image forming apparatus forms, on the belt, a registration pattern (i.e., a pattern used for alignment). The registration pattern includes a plurality of mark pairs, each of which includes a mark of a reference color (e.g., black) and a mark of an adjustive color (e.g. cyan, magenta or yellow). The mark pairs differ from one another in shift amount of the adjustive-color mark from the reference-color mark.
The density of each mark pair depends on the shift amount. Specifically, the density of each mark pair actually formed on the belt depends on the actual shift amount, which includes the above designed shift amount and a displacement amount of the adjustive-color mark from the reference-color mark.
The densities of mark pairs actually formed on the belt are sequentially measured by a density sensor, so that the mark pair having the highest density can be identified. The actual shift amount associated with the identified mark pair can be considered to be zero.
Therefore an amount corresponding to the designed shift amount associated with the identified mark pair can be determined as an estimated displacement amount of an image to be formed of an adjustive color from an image to be formed of a reference color. Then, an image forming position is adjusted so that the estimated displacement amount is canceled.
However, a light sensitive signal, which is outputted from the density sensor and indicates the detected density of an image, may temporarily be affected by noise. In this case, a mark pair not having the highest density may be incorrectly identified as a mark pair having the highest density, due to the noise.
That is, the actual shift amount associated with the identified mark pair may fail to be zero, resulting in inaccuracy of an estimated displacement amount to be determined based on the identified mark pair.